Proteins are involved in a variety of physiological and biochemical pathways within organisms. In particular, proteins are potent yet specific transducers in myriad processes which influence aspects of disease formation, defense, and immunity. For example, cytokines, a specific family of proteins, are heavily involved in cellular signaling and trafficking within the immune system. When cytokines (e.g., interleukin 6 (IL-6) or tumor necrosis factor alpha (TNF-α)) are released by cells, they target and bind to specific receptors on cellular membranes. Upon receptor binding, a cascade of intercellular signaling events occurs, ultimately resulting in changes in cellular behavior. In the case of IL-6 and TNF-α, release triggers numerous processes, including inflammatory response.
Due to their specificity and their role in many biochemical pathways, proteins can serve as molecular targets in therapeutic and diagnostic applications (e.g., oncology, transplant rejection, and inflammation). For example, a growing number of proteins have been identified as biomarkers for disease. In some cases, biomarkers manifest themselves early in a disease process, providing an early indicator for a disease state. For example, monokine induced by interferon γ (MIG, CXCL9) has been implicated as a key biomarker which predicts allograft rejection in both murine models and in humans. The detection of such biomarkers offers the potential to improve patient outcomes through earlier and more targeted intervention.
Existing clinical methodologies for identifying and quantifying proteins suffer from significant drawbacks. Enzyme-linked immunosorbent assays (ELISAs) are the most widely used means for protein detection in clinical settings. While ELISAs can effectively detect proteins in vitro, ELISAs are labor intensive, utilize multiple reagents, and require hours to perform. ELISAs are also unable to assay analyte content in vivo or in situ.
Thus, there remains a need for sensitive and efficient means of detecting and/or quantifying analytes which are inexpensive, detect analytes without labels or additional reagents, exhibit exponential responses to surface potential changes mediated by analyte binding, require limited sample preparation, and operate in real-time. In addition, direct assays that can be utilized to detect analytes, including proteins, in vivo or in situ offer the opportunity to address the challenges and costs associated with treating numerous diseases, including transplant rejection, cancer, and inflammation.